Excess morbidity and mortality in the US ESRD population compared to other industrialized nations in Europe and Asia is a subject of regulatory concern and active ongoing investigation. There is evidence that a large proportion, perhaps the majority, of American hemodialysis patients receive inadequate therapy and that this may be the cause of high mortality. Children on hemodialysis suffer many of the same problems of adult patients and yet few investigations have targeted pediatric dialysis patients. Several mathematical models have been developed to describe the events that occur during and after hemodialysis treatments and to measure accurately the effective clearance of treatments. Available data suggest that models may be especially useful in measuring dialysis delivery at extremes of body size, and thus would be particularly important in small children. However, these models have been validated only in adult hemodialysis patients and have not been adequately studied in children. In this protocol, mathematical modeling of hemodialysis will be performed in children during routine dialysis sessions. In addition, this study will obtain normative data on body water spaces (total body water, intracellular water, and extracellular water) in children with chronic kidney failure to allow accurate modeling of clearances during dialysis. Data on body water spaces in children with renal failure will also be important for pharmacokinetic applications to allow predictions of drug distribution and clearances.